This invention relates to a process of electroplating substrates and more particularly to a process of electroplating porous substrates so that the pores of the substrates remain open and unclogged.
Electroplating is a well known technique of depositing metal or alloy from a suitable electrolyte solution onto an electrically conducting article. More generally, the article is connected to a negative current source so as to act as the cathode in the electrolyte solution. Hydrated positive metal or alloy ions which are dissolved or complexed in the solution are attracted to the cathode by the negative current along electrostatic lines of force present in the solution. As the ion approaches the vicinity of the cathode, it progressively sheds the hydration envelope and becomes associated with the delocalized charge on the article. At the same time, the ion crosses the so-called "double layer" and loses some of its effective charge to become a partially neutral atom or adion, which is then deposited on the surface of the article. The adion then diffuses along this surface to a nearby lattice growth plane, where it becomes permanently fixed in position.
Lattice growth planes are determined by the submicroscopic morphology of the article and its plating history. As the metallic coating builds up, the adion gradually proceeds to a state of full dehydration and complete incorporation into the crystal structure now coating the surface of the article. The plating process may be halted at any time to provide a coating of any desired thickness. An anode consisting of the metal or alloy to be deposited and immersed in the electrolyte solution may be used to provide replacement ions for those depleted from the electrolyte through a similar, though reversed, process. The current supplied by the external source provides the necessary electron transfer to create and subsequently discharge the metal or alloy ions.
The electroplating process has proved very advantageous in depositing very thin and uniform layers or metal or alloy onto substrates. However, it is apparent that if this process were directly applied to depositing material onto a porous substrate, the material to be deposited could, depending upon the electroplating time, clog the pores of the substrate. For certain uses of such plated porous substrates, this would be undesirable. For example, if the porous substrate were to be used as an electrode in an electrolysis process, it may be important that fluid communication through the substrate be maintained and, of course, if the pores were clogged, this would not be possible. As another example, if the porous substrate were to be used as a filter in a corrosive environment, it would be desirable to plate the substrate with protective material while maintaining the porosity of the substrate.